docs: def get_default_id(credentials=None):
      """ Get default project id.
  
      Returns: the default project id if there is one, or None.
      """
      project_id = _utils.get_project_id()
      if project_id is None:
        projects, _ = Projects(credentials)._retrieve_projects(None, 2)
        if len(projects) == 1:
          project_id = projects[0].id
      return project_id